The present invention is directed to the utilization of Na.sub.2 SO.sub.4 . 10H.sub.2 O (Glauber's salt) as the liquid-solid phase change material in the storage of thermal energy.
The invention is particularly applicable to the utilization of Glauber's salt in a thermal energy storage device, which imparts interparticle motion to crystals in the system. Such a device is described in pending U.S. patent application Ser. No. 706,875 - Herrick, filed July 19, 1976, and assigned to the assignee of the instant invention. Therein a liquid-solid phase change material is sealed in a container, which is slowly rotated about a generally horizontal axis at a substantially constant rotational speed. The aforementioned application is incorporated by reference.
Much of the work that has been done in modifying the growth habit of various crystalline materials is set forth in the textbook "Crystal Growth" by H. E. Buckley [John Wiley & Sons, Inc., Third Printing (1956)] in Chapter 10 "Crystal Habit Modification by Impurities", pages 339-387. The statement is found therein beginning at the bottom of page 344 that "The use of iron as an impurity in the growth of large ammonium dihydrogen phosphate (ADP) crystals to stimulate prism growth is quite a common practice." It is also pointed out therein that the growth habit of ammonium sulfate can be changed to produce stouter crystals by the use of ferric ion and lowered pH. However, in contrast to earlier work in which the objective was to stimulate growth or produce larger crystals of Glauber's salt, the present invention has the goal of preventing the growth of large crystals.
In addition to the use of metal ions as impurities to affect crystal habit, a wide range of dyes and surfactants have been reported as having various effects in a number of crystallizing systems. A number of dyes and surfactants were employed as impurities in the rotating Glauber's salt system, but none had the desired effect of limiting crystal growth to small sizes.